There are three major types of brushless electric machines available for the electric vehicle (HV) and hybrid electric vehicle (HEV) drive systems. These are the induction machine, the PM machine, and the switched-reluctance machine.
Permanent magnet (PM) machines have been recognized for having a high power density characteristic. A PM rotor does not generate copper losses. One drawback of the PM motor for the above-mentioned application is that the air gap flux produced by the PM rotor is limited, and therefore, a sophisticated approach is required for high speed, field weakening operation. Another constraint is that inductance is low, which means that current ripple must be controlled.
It is understood by those skilled in the art that a PM electric machine has the property of high efficiency and high power density, however, the air gap flux density of a PM machine is limited by the PM material, which is normally about 0.8 Teslas and below. A PM machine cannot operate at an air gap flux density as high as that of a switched reluctance machine. When the PM motor needs a weaker field with a reasonably good current waveform for high-speed operation, a sophisticated power electronics inverter is required.
Rosenberg et al., U.S. Pat. No. 3,411,027, illustrates a permanent magnet (PM) machine with field excitation. There is no intent to shield the flux path for the field induced flux to prevent flux leakage. Therefore, significant flux leakage would occur resulting in a reduction of power density. In addition, Rosenberg et al. does not teach any additional reluctance poles or reluctance flux paths for producing reluctance torque in such a PM motor.
Koharagi et al. U.S. Pat. No. 6,441,525, discloses permanent magnet (PM) barriers arranged in a V-shape and a U-shape. This patent also teaches the additional reluctance flux paths of a rotor, but without using a shunt field excitation.
It is known in the art that the reluctance torque is produced by the difference between the d-axis inductance, Ld, and the q-axis inductance, Lq. As this difference increases, the reluctance torque increases. The flux passing through the d-axis poles in Koharagi is concentrated, but is also reduced, by the V-shaped and U-shaped PM barriers. This means that the q-axis inductance, Lq, produces a flux having a less restrictive flux path to pass through along the q-axis, compared with the flux produced by the d-axis inductance, Ld. The above d-axis and q-axis flux paths are restricted to two dimensions and without the additional excitation flux paths disclosed herein.
Tajima et al., U.S. Patent Pub. No. U.S. 2005/0200223 utilizes PMs arranged in a V-shape to reduce the core loss under certain operating conditions. There are several drawbacks with this approach. Because the back emf produced by the strong PMs is proportional to the speed, a boost converter is needed to raise the voltage fed to the motor at high speed. The core loss at high speed is also high, due to the strong PMs, and this is true even when the motor is disconnected from the power supply. This publication does not disclose the use of field excitation to improve performance.
Tajima also discloses symmetrical concavities per pole formed on the air gap face of the magnetic pole pieces of the rotor iron core. These are provided for the purpose of reducing core losses otherwise inherent in the design.
Hsu, U.S. Pat. No. 6,972,504, issued Dec. 6, 2005, discloses a PM machine with reluctance poles and DC excitation coils positioned at opposite ends of the rotor. Prior designs have been largely symmetrical in their configuration of the PM poles and reluctance poles.
The present invention is intended to improve reluctance torque and power density in a PM machine, while still providing a compact configuration.